Exhaust system members, such as an exhaust manifold of an automobile and the like, are passed through by a high-temperature exhaust gas discharged from an engine, and thus a material that constitutes the exhaust system members is required to have various properties such as high-temperature strength, oxidation resistance, good thermal fatigue properties, and the like. Ferritic stainless steel having excellent heat resistance has been used for the exhaust members.
The exhaust gas temperature varies depending on the model of vehicle. In recent years, the exhaust gas temperature has been approximately in a range of 800° C. to 900° in most vehicles, and the temperature of an exhaust manifold through which the high-temperature exhaust gas discharged from the engine passes increases to be in a range of 750° C. to 850° C. which is high. However, new enforcement of restrictions on exhaust gas, and an improvement of fuel efficiency have progressed with the recent surge of environmental problems, and it is considered that the temperature of the exhaust gas may increase to be as high as approximately 1000° C.
As the ferritic stainless steel which has been used recently, SUS429 (Nb—Si added steel) and SUS444 (Nb-MO added steel) are exemplary examples, and the high-temperature strength is improved due to the addition of Si and Mo on the basis of the addition of Nb. Since SUS444 contains approximately 2% of Mo, SUS444 has the highest strength. However, SUS444 cannot be utilized at high temperatures in which the temperature of the exhaust gas is higher than 900° C., and thus there is a demand for ferritic stainless steel having a heat resistance equal to or higher than that of SUS444.
Various materials for exhaust system members have been developed to cope with the demand. For example, in Patent Document 1, in order to improve the thermal fatigue properties, a method is studied which controls the number of Cu phases having major axes of 0.5 μm or greater to be in a range of 10 pieces/25 μm2 or less, and which controls the number of Nb compound phases having major axes of 0.5 μm or greater to be in a range of 10 pieces/25 μm2 or less. However, only coarse precipitates of a Laves phase and an ε-Cu phase are considered, and precipitates having sizes of 0.5 μm or less are not disclosed. Patent Documents 2 and 3 disclose a method of obtaining solid-solution strengthening of Cu and precipitation strengthening due to an ε-Cu phase in addition to solid-solution strengthening of Nb and Mo by defining the amount of precipitates; and thereby, high-temperature strength equal to or higher than that of SUS444 is accomplished. However, the thermal fatigue properties are not disclosed. Patent Documents 5 and 6 disclose technologies in which W is added together with Nb, Mo, and Cu. Patent Document 5 discloses a method of utilizing solid-solution strengthening of Cu, Nb, Mo, and W, but Patent Document 5 does not disclose a thermal fatigue lifetime. Patent Document 6 discloses a method in which compounds of Fe and P are utilized as precipitation sites to allow a Laves phase and a ε-Cu to minutely precipitate into a grain; and thereby, the strength stability of precipitation strengthening and the thermal fatigue lifetime at 950° C. are improved. However, with regard to the thermal fatigue lifetime, 2000 cycles or more are determined as “passing”, and an examination on the thermal fatigue lifetime for a longer period of time is not made.
Most recently, Patent Document 7 discloses a technology in which a Nb carbonitride is used in addition to a Laves phase so as to maintain solid-solution strengthening of Nb and Mo, and an excellent thermal fatigue lifetime (1500 cycles or more) at 950° C. is obtained by an effect of minutely dispersing a Laves phase and an ε-Cu phase due to B.